The necessity of sterilizing surfaces for health and sanitary purposes has long been recognized. Effective sterilization processes are needed for a variety of purposes including aseptic packaging, medical instrument sterilization, biocidal vector environmental remediation, fumigation, vessel sterilization, food stuff treatments, and others.
Among the compounds employed as a sterilizer for such uses is peracetic acid (“PAA”), also called peroxyacetic acid. In order to prolong the shelf-life of PAA compositions, stabilizers are typically added. These stabilizers act as chelators for dissolved metal cations that can disrupt the peroxide bond.
The use of vapor phase peracetic acid to sterilize surfaces is described in U.S. Patent Application 2010/0196197. This publication discloses the use of peracetic acid stabilized with phosphoric acid or acid stabilizers such as Dequest 2010, 1-Hydroxyethylidene-1,1,-diphosphonic acid. While such process is effective to sterilize a variety of different surfaces, it has been found that when such phosphonic acid stabilizers are employed the surface of the heating element used to vaporize the peracetic acid solution tends to become covered with residue over a period of time. This build-up of residue requires that such surface be periodically cleaned, a process which can be time consuming and expensive.
It would therefore be highly desirable to possess a method for using vapor phase peroxyacetic acid as a sterilizing agent which method did not require the relatively frequent cleaning of the heating surfaces of the equipment employed.
While PAA compositions comprising citric acid have been disclosed in the past, such compositions have been employed in liquid aqueous sanitizing applications only. Thus, for example, WO 2008/079999 discloses sterilizing compositions comprised of PAA, citric acid or a salt thereof, and salicylic acid or a salt thereof, which compositions are applied topically to the surfaces to be disinfected employing solution spray systems or the like. Somewhat similarly, U.S. Pat. No. 6,117,457 and European Patent Application 720814 both disclose the use of citric acid in aqueous PAA generation systems; either as a pH regulator in fish ponds or as a means of providing extended PAA generation, respectively.
However, it has now been unexpectedly found that when PAA stabilized with an organic acid such as citric acid is employed in vapor phase sterilization, the buildup of residue on heating surfaces is considerably less than when a phosphonic acid stabilizer is employed. This result is particularly unexpected in view of the showing that aqueous solutions of citric acid (in the absence of PAA) result in the undesirable buildup of residue on heating surfaces.